/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.tomcat.util.http.fileupload.util.mime;

import java.io.IOException;
import java.io.OutputStream;

/**
 * @since 1.3
 */
final class QuotedPrintableDecoder {

	/**
	 * The shift value required to create the upper nibble
	 * from the first of 2 byte values converted from ascii hex.
	 */
	private static final int UPPER_NIBBLE_SHIFT = Byte.SIZE / 2;

	/**
	 * Hidden constructor, this class must not be instantiated.
	 */
	private QuotedPrintableDecoder() {
		// do nothing
	}

	/**
	 * Decode the encoded byte data writing it to the given output stream.
	 *
	 * @param data The array of byte data to decode.
	 * @param out  The output stream used to return the decoded data.
	 * @return the number of bytes produced.
	 * @throws IOException if a problem occurs during either decoding or
	 *                     writing to the stream
	 */
	public static int decode(byte[] data, OutputStream out) throws IOException {
		int off = 0;
		int length = data.length;
		int endOffset = off + length;
		int bytesWritten = 0;

		while (off < endOffset) {
			byte ch = data[off++];

			// space characters were translated to '_' on encode, so we need to translate them back.
			if (ch == '_') {
				out.write(' ');
			} else if (ch == '=') {
				// we found an encoded character.  Reduce the 3 char sequence to one.
				// but first, make sure we have two characters to work with.
				if (off + 1 >= endOffset) {
					throw new IOException("Invalid quoted printable encoding; truncated escape sequence");
				}

				byte b1 = data[off++];
				byte b2 = data[off++];

				// we've found an encoded carriage return.  The next char needs to be a newline
				if (b1 == '\r') {
					if (b2 != '\n') {
						throw new IOException("Invalid quoted printable encoding; CR must be followed by LF");
					}
					// this was a soft linebreak inserted by the encoding.  We just toss this away
					// on decode.
				} else {
					// this is a hex pair we need to convert back to a single byte.
					int c1 = hexToBinary(b1);
					int c2 = hexToBinary(b2);
					out.write((c1 << UPPER_NIBBLE_SHIFT) | c2);
					// 3 bytes in, one byte out
					bytesWritten++;
				}
			} else {
				// simple character, just write it out.
				out.write(ch);
				bytesWritten++;
			}
		}

		return bytesWritten;
	}

	/**
	 * Convert a hex digit to the binary value it represents.
	 *
	 * @param b the ascii hex byte to convert (0-0, A-F, a-f)
	 * @return the int value of the hex byte, 0-15
	 * @throws IOException if the byte is not a valid hex digit.
	 */
	private static int hexToBinary(final byte b) throws IOException {
		// CHECKSTYLE IGNORE MagicNumber FOR NEXT 1 LINE
		final int i = Character.digit((char) b, 16);
		if (i == -1) {
			throw new IOException("Invalid quoted printable encoding: not a valid hex digit: " + b);
		}
		return i;
	}

}
